The overall objective of this study is to further our understanding of the role of the S. aureus capsule in bacterial pathogenesis. Because S. aureus produces so many factors that contribute to its virulence, we willl study transposon-derived strains which are isogenic except for their degree of encapsulation. We have shown that a highly encapsulated strain of S. aureus is more virulent in mice than two strains that produce small capsules: the parental strain and a transposon-dreived mutant strain. We now proposed to isolate an isogenic, unencapsulated strain by transposon mutagenesis. The relative virulence of this strain will be assessed by mouse infection studies of bacteremia, abscess formation, and lethality. S. aureus strains which produce small capsules are closely resemble clinical isolates than mucoid stains, yet the small capsules have not been adequately characterized biologically or biochemically. Therefore, we will purify capsular polysaccharides from two nonmucoid stains that elaborate small capsules and compare them biochemically and serologically to the capsule produced by the mucoid, highly encapsulated strain. Immunologic parameters which protect healthy individuals from S. aureus infections are poorly understood. We have shown that mice immunized with pruified capsular polysacharide are protected from renal abscess formation induced by the homologous bacterial strain. We plan to extend these studies by evaluating whether immunization affords protection against abscess formation, bacteremia, and lethality from mucoid and nonmucoid S. aureus strains of the same capsular type. Using an in vitro opsonophaocytic assay, we plan to examine how stains that differ quantitatively in cpasule production are killed by human polymorphonuclear leukocytes. Monoclonal and polyclonal antibodies to the S. aureus capsule, teichoic acid, and peptidoglycan will be used in this assay to define the requirements for opsonic killing of these bacterial strains.